The invention relates to a transmitter for transmitting a wideband digital information signal, the transmitter comprising:
an input terminal for receiving the wideband digital information signal, PA1 signal splitter means for splitting the digital information signal into M sub signals, each one of the sub signals being representative of a component of the wideband digital information signal which is present in a corresponding one of M adjacent narrow bands in the frequency band of the wideband digital information signal, where M is an integer larger than 1, PA1 quantizing means for quantizing samples comprised in a macroblock, a macroblock comprising the samples present in time equivalent signal blocks comprised in the sub signals, one signal block in a sub signal, each signal block comprising q of the samples of a sub signal, the quantizing means being adapted to quantize the samples present in said macroblock into quantized samples in response to bit allocation information supplied to the quantizing means so as to obtain a quantized macroblock comprising corresponding time equivalent blocks of quantized samples, PA1 bit allocation information derivation means for deriving bit allocation information for subsequent macroblocks, the bit allocation information comprising for each of the time equivalent signal blocks in a macroblock a bit allocation value representative of the number of bits with which the q samples in a signal block will be represented after quantization in the quantizing means, the bit allocation information derivation means being adapted to derive the bit allocation information for a macroblock in response to a number B of bits which is available for quantization of the samples in the macroblock so as to obtain the quantized macroblock of quantized samples, PA1 formatter means for combining the quantized samples comprised in a quantized macroblock into a digital output signal having a format suitable for transmission, to a method of transmitting said wideband digital information signal and to a receiver for receiving the wideband digital information signal. The wideband digital information signal can be an wideband digital audio signal. PA1 a buffer memory for storing quantized samples, PA1 filling degree detection means for detecting the filling degree of the buffer memory and generating a filling degree detection signal in response to said filling degree, the bit allocation information derivation means being provided with a control signal input for receiving the filling degree detection signal. PA1 receiving the wideband digital information signal, PA1 splitting the digital information signal into M sub signals, each one of the sub signals being representative of a component of the wideband digital information signal which is present in a corresponding one of M adjacent narrow bands in the frequency band of the wideband digital information signal, where M is an integer larger than 1, PA1 quantizing samples comprised in a macroblock, a macroblock comprising the samples present in time equivalent signal blocks comprised in the sub signals, one signal block in a sub signal, each signal block comprising q of the samples of a sub signal, the quantizing means being adapted to quantize the samples present in said macroblock into quantized samples in response to bit allocation information supplied to the quantizing means so as to obtain a quantized macroblock comprising corresponding time equivalent blocks of quantized samples, PA1 deriving bit allocation information for subsequent macroblocks, the bit allocation information comprising for each of the time equivalent signal blocks in a macroblock a bit allocation value representative of the number of bits with which the q samples in a signal block will be represented after quantization in the quantizing means, by deriving the bit allocation information for a macroblock in response to a number B of bits which is available for quantization of the samples in the macroblock so as to obtain the quantized macroblock of quantized samples, PA1 combining the quantized samples comprised in a quantized macroblock and the corresponding bit allocation information into a digital output signal having a format suitable for transmission, is characterized in that the method further comprises the steps of PA1 storing quantized samples in a buffer memory, PA1 detecting a filling degree of the buffer memory and generating a filling degree detection signal in response to said filling degree, PA1 the bit allocation information derivation step further comprising the substep of receiving the filing degree detection signal.
A transmitter as defined in the opening paragraph is known from EP-A 457,390 and EP-A 457,391, the documents (D1) and (D2) respectively, in the list of references given below. More specifically, in a bit allocation step, the powers in each of the subbands are calculated by squaring the sample values present in time equivalent signal blocks of the subband signals and summing the squared sample values in a time equivalent signal block. The signal blocks in the documents listed above are of constant length and are 12 samples long. The powers thus obtained are processed in a processing step in which use is made of a psycho acoustic model so as to obtain masked threshold values. Another way of obtaining the masked threshold values is by carrying out separately a Fourier transform on the wideband digital information signal and applying the psycho acoustic model on the Fourier Transform results. The masked threshold values, together with the scale factor information, result in bitneeds b.sub.1 to b.sub.M for the samples in the time equivalent signal blocks of the M subband signals. Next, those bitneed values are used so as to allocate B bits, that are available in a bitpool of B bits, to the samples, resulting in the bitallocation information values n.sub.1 to n.sub.M, n.sub.m indicating the number of bits with which each of the 12 samples in the signal block of subband m are represented, after having carried out a quantization on the samples in the subbands.
In the prior art transmitter, which receives a wideband digital information signal sampled with a sampling frequency of 48 kHz, the total frequency band to be encoded is 24 kHz. This frequency band is split into 32 narrow bands of equal width, so that they have a constant width of substantially 750 Hz each. For transmitting speech signals, the frequency band of interest of the wideband digital information signal may be 4 kHz wide, which may be split into e.g. four narrow bands.
In the formatter means, the quantized samples (and generally also the bit allocation information) are combined and converted into an output signal suitable for transmission. The output signal is transmitted with a substantially constant bitrate.
Investigations have resulted in the knowledge that the output signal may sometimes be distorted.